1. Field of the Invention
The present invention relates to a cellular mobile communication system in which a service area is divided into plural cells, and a mobile station residing within a cell and a base station that forms the cell establish communication by sharing a frequency band assigned to the system according to a multiple access scheme. The present invention particularly relates to a TDMA-FDM cellular mobile communication system that assigns to each cell a set of plural narrow band frequency channels of the frequency band assigned to the system, and adjusts the assignment of the frequency channels such that the same or adjacent frequency channel is only assigned to cells that are separately located. The present invention also relates to a wireless control apparatus and a communication method used in a CDMA cellular mobile communication system in which the frequency band assigned to the system is shared by all users of the cells according to the code division multiple access scheme.
2. Description of the Related Art
Wireless signals transmitted in space over the ground are gradually attenuated as the distance from the transmission point is increased due to energy dispersion in the propagation process, absorption by plants, and shielding by land features and land objects, for example. The extent of the attenuation may be represented as propagation loss indicating the ratio between the transmission power at the transmission point and the reception power at a measuring point.
It is generally known that in an urban environment, the propagation loss may be increased by 8 to 16 times when the propagation distance is doubled. Accordingly, considering the fact that there is a physical lower limit for a thermal noise power level of a receiver and a determination of whether a wireless signal may be received is made based on the relative ratio between the reception power and the thermal noise power, it may be understood that the reception enabling distance at which reception can be realized is controlled by the maximum value of transmission power of a transmitter.
In a cellular mobile communication system, a relatively wide service area is divided into cells corresponding to relatively small areas, and a base station assigned to each cell is configured to establish communication with a mobile station residing within the relevant cell. Since the reception enabling distance of a wireless signal is controlled by the maximum transmission power of a transmitter, if a wide service area were not divided into cells so that the service are is handled by one base station, the base station has to have a large transmission power that is impossible to realize. It is noted cells are preferably arranged to be smaller; however, the number of cells covering the service area and the number of base stations have to be increased in such a case.
In a cellular mobile communication system that is assigned a limited frequency band, the system capacity, namely, the number of users that may be simultaneously accommodated, may be increased by reducing the area of cells. For example, in a system with low interference resistance that uses narrow band signals based on the time division multiple access scheme or the frequency division multiple access scheme, the quality of communications that are established simultaneously within an electromagnetically shared space using the same frequency or adjacent frequencies may be immediately degraded due to crosstalk.
Therefore, in a mobile communication system having a service area that is realized by a high position antenna and a high power transmitter where the service area is not divided into plural cells, frequency channels are aligned at the smallest intervals for avoiding crosstalk, and the system capacity reaches its limit once narrow band signals are assigned to all the frequency channels. In this system, the system capacity of the overall service area is fixed regardless of the size of the service area.
However, even in a system using narrow band signals where crosstalk immediately leads to communication quality degradation and disconnection, if the service area is divided into plural cells using low position antennas and low power transmitters to realize a cellular mobile communication system, frequencies may be repetitively used within the service area without causing crosstalk. For example, in a cellular mobile communication system, adjustments may be made to avoid using the same frequency or adjacent frequencies in neighboring cells of a current cell (e.g., adjacent cells and cells positioned one cell apart from the current cell) that share an electromagnetic space with the current cell to a great extent and use the same frequency or the adjacent frequencies in other cells that do not share an electromagnetic space with the current cell to such a great extent.
Thus, by increasing the number of cells into which a service area is divided in accordance with the enlargement of the service area, the overall system capacity of the service area may be increased.
For example, in a cellular mobile communication system that uses narrow band signals based on PDC (FDMA-TDM), for example, a set of narrow band frequency channels is assigned to each cell, and adjustments are made so that the same frequency or adjacent frequencies are only assigned to cells that are adequately spaced apart from each other.
In the multiple access scheme, a wide band signal may be used to secure resistance to crosstalk and prevent immediate degradation in communication quality even when the same frequency band is shared by users in adjacent cells upon establishing communication. In the following the code division multiple access scheme (CDMA) is described to illustrate that even in a mobile communication system based on a multiple access scheme, the overall system capacity of the service area may be increased through cell division as with the system using narrow band signals.
In the code division multiple access scheme, individual communication waves may be detected, and the same frequency band may be shared by plural communication waves through frequency spreading using spreading codes (unique sequence codes) to enable mutual distinction between the communication waves.
For example, in the downlink (uplink) of a W-CDMA cellular system, even when one base station (mobile station) simultaneously establishes plural channels for communication, different spreading codes (channelization codes) are used to spread the channels before multiplexing the channels at the base station (mobile station) so that the channels may be distinguished at the reception side mobile station (base station).
Also, in order to enable repeated use of the limited number of channelization codes within the system among different base stations (mobile stations), a different spreading code (scrambling code) is used for each base station (mobile station) to spread the multiplexed signal further upon transmitting the signal. In this case, plural channels from plural base stations (mobile stations) are simultaneously received within the same frequency band at the mobile station (base station) corresponding to the reception side of the downlink (uplink). However, under predetermined conditions, the mobile station (base station) may identify a group of channels addressed to itself and the individual channels assigned to each communication.
It is noted that the number of communication waves that may simultaneously share the same frequency band while maintaining an adequate communication quality, namely, the system capacity, is limited.
Specifically, in the downlink (uplink), adequate communication quality is required in the channels subject to demodulation in order to detect/identify in each communication the plural channels from plural base stations (mobile stations) that are simultaneously received in the same frequency band at the mobile station (base station) and accurately demodulate the received channels.
In a case where the wireless propagation environment is fixed, the communication quality of a channel may be determined by the signal-to-interference ratio (SIR) that takes into account the reception signal power and the processing gain of the channel subject to demodulation. An interference noise power is a total power of interference from a delay wave of the channel subject to demodulation that has reached the receiver, interference from other channels, and the receiver thermal noise. The interference noise power may be reduced to a fraction of the spreading code length per information bit by the processing gain after an inverse spreading process for recovering the spread channel is performed.
The interference from other channels includes interference from the cell transmitting the channel to be demodulated and interference from cells other than the cell transmitting the channel to be demodulated.
It is noted that the channel to be demodulated may similarly correspond to interference when viewed from the other channels. Accordingly, the base station (mobile station) may not transmit a channel with needlessly high transmission power to increase the reception signal power at the mobile station (base station) and obtain a communication quality that is higher than the required communication quality. In other words, the mobile station (base station) controls the transmission power of the base station (mobile station) at high speed (high speed transmission power control) so that communication may be established with the minimum required transmission power to obtain adequate communication quality with respect to the wireless propagation environment.
As the number of communications that are simultaneously established within the same frequency band is increased, the total reception power level in the mobile station (base station) receiver increases. This owes to the fact that as the number of communication channels is increased, the interference noise power increases, and the base station (mobile station) performing high speed transmission power control increases the transmission power to secure the required SIR for each channel. This in turn leads to further increase in the interference noise power.
In a case where the maximum transmission power of the base station (mobile station) is sufficiently high, appropriate measures may be implemented in response to the increase in the interference noise power. Thus, in a case where the transmission power may be limitlessly increased, the number of simultaneous communication channels may be increased up to a number close to a logically obtained marginal capacity (pole capacity). However, in practice, limits are imposed with respect to the total transmission power of the base station (mobile station) and the transmission power that may be assigned to each channel.
Therefore, in the downlink, transmission powers reach the upper limit starting from channels established with respect to mobile stations with relatively large propagation loss from the base station so that adequate communication quality may not be obtained. As a result, channels in communication may be forcedly disconnected, and adequate transmission power may not be assigned to a channel newly established with respect to a mobile station with which communication is desired.
Also, in the uplink, transmission powers to be assigned to channels reach the upper limit starting from mobile stations with large propagation loss from the base station so that adequate communication quality may not be obtained. As a result, channels in communication may be forcedly disconnected, and a connection request of a mobile station with which communication is desired may not reach the base station with adequate communication quality.
The above described phenomenon is also referred to as cell breathing since it brings about effects of virtually reducing the cell coverage. In a system such as the cellular system where a planar service area is required, cells need to be successively arranged to adequately overlap each other. Accordingly, when cell breathing is about to occur (e.g., when a gap is about to be created between cells), a new connection request may be rejected (call acceptance control), or a channel in communication may be forcedly disconnected (congestion control) in order to prevent the occurrence of cell breathing.
Accordingly, in the case where limits are imposed with respect to the maximum transmission powers of the base station and the mobile station, the system capacity is controlled by cell coverage. Even if limits are not imposed on the maximum transmission powers of the base station and the mobile station, it is theoretically impossible to accommodate a number of simultaneous communication channels exceeding the pole capacity. For example, the system capacity of a 5 MHz frequency band (3.84×1.22 MHz width+margins) corresponding to a basic frequency band for downlink and uplink in the W-CDMA system is about 100 at maximum for audio communication channels.
As can be appreciated, in the code division multiple access scheme, a system capacity upper limit regulated by the pole capacity may be imposed with respect to the frequency band assigned to a system. Also, in a case where a service area is not divided into cells, the overall system capacity of the service area is limited to be less than or equal to the pole capacity per cell regardless of the size of the service area.
Also, in the code division multiple access scheme, the area of cell coverage and the system capacity are in a tradeoff relationship when a limit is imposed on the transmission power of the transmitter. For example, when the cell coverage is large, the system capacity per cell is reduced. Also, in the case where the service area is not divided into cells, the system capacity of the entire service area is further limited by the transmission power of the transmitter.
On the other hand, as the area of cells of a service area is reduced, the system capacity may be gradually freed from restrictions imposed by the transmission power of the transmitter. As a result, the system capacity of a cell may be closer to the pole capacity, and the overall system capacity of the service area may be increased by increasing the number of cells. In other words, by increasing the number of cells in accordance with the enlargement of the service area, the system capacity may be increased.
However, it is noted that by dividing the service area into cells, interference may be generated from other cells due to the use of the same frequency band by plural adjacent cells. Therefore, the system capacity of the service area does not correspond to the number obtained by multiplying the system capacity of each cell by the cell division number where interference of the other cells is not considered.
As can be appreciated from the above descriptions, in the time division access scheme and the frequency division access scheme where relatively narrow band signals are used, although interference resistance may be low, the same frequency or adjacent frequencies are used in segregated cells so that the system capacity of the service area may be increased in accordance with the enlargement of the service area by increasing the cell division number.
Also, in the code division access scheme, wide band signals are used so that high interference resistance may be realized, and users residing within adjacent cells may share the same frequency band so that the system capacity of the service area may be increased in accordance with the enlargement of the service area by increasing the cell division number.
In the following, an example is described in which restrictions based on the transmission power of the transmitter that are imposed on the system capacity can be disregarded by reducing the area of the cell. In a system that uses narrow band signals, the system capacity of the service area is constrained by the fact that the repeated distance of the same frequency or adjacent frequencies may not be reduced. This is because a cell in which the same frequency or adjacent frequencies is used has to be segregated from the current cell so that the cells do not share an electromagnetic space to such a great extent.
In a system that uses wide band signals, interference from other cells such as adjacent cells and neighboring cells using the same frequency band may leak into the current cell.
In this regard, by taking measures to reduce the sharing of electromagnetic space between the current cell and its neighboring cells, interference from other cells may be reduced and the system capacity in an assigned frequency band may possibly be increased. It is noted that example of such measures include narrowing the beam of a vertical pattern of a base station antenna to realize appropriate beam tilting, and appropriately directing a horizontal pattern into a beam as is described below.
In a vertical pattern that is arranged into a beam, a relatively high gain is obtained in the main lobe direction whereas the gain in the side lobe direction is relatively low. Accordingly, the main lobe of he vertical (horizontal) pattern of the base station antenna may be directed to the cell administered by the relevant base station and the side lobe may be directed to the cells administered by neighboring base stations providing service using the same frequency band (beam tilting) to thereby reduce the sharing of electromagnetic space between cells.
Consequently, the distance required between cells for adequately avoiding crosstalk may be reduced in a system using the time division multiple access scheme or the frequency division multiple access scheme, and the same frequency or adjacent frequencies may be repeatedly used in cells arranged closer to each other.
Also, in a system using the code division access scheme, beam tilting may produce an effect of reducing the interference from other cells. In this way, the system capacity of a cell in an assigned frequency band may be closer to the pole capacity.
Beam tilting using a vertical (horizontal) pattern of a base station antenna for maximizing the system capacity in an assigned frequency band may be determined by an appropriate half with and a tilt angle (orientation angle).
It is noted that the half width corresponds to an angle formed by a direction with a gain of no more than 3 dB with respect to the maximum gain of the vertical (horizontal) pattern (i.e., main lobe width) and represents the extent to which the beam is narrowed.
The tilt angle corresponds to an angle forming a horizontal direction with respect to the maximum gain direction of the vertical pattern, the tilt angle being represented by a positive number when the maximum gain direction is oriented toward the ground. The orientation angle corresponds to an angle that forms a true north direction with respect to the maximum gain direction of the horizontal pattern.
In a case where the cell arrangement or the cell boarder is fixed beforehand by the cell design, the following qualitative rules apply with respect to optimal beam tilting. Namely, adequate antenna gain may be secured in the main lobe direction (normally at cell border) in a case where the vertical half width of the antenna is small in a base station realizing a cell width a small cell radius (distance between base station and cell boarder). On the other hand, the required antenna gain may not be secured within the cell since the side lobe is oriented in the direction with an anticipated angle greater than the main lobe direction.
In contrast, an adequate antenna gain may be secured within the cell in a case where the vertical pattern half width of the antenna is large in a base station forming a cell with a large cell radius. On the other hand, the required antenna gain may not be secured in the main lobe direction (normally cell boarder).
Also, when the tilt angle of the antenna vertical pattern in the base station is small, the orientation direction of the main lobe becomes close to horizontal so that the antenna gain is increased with respect areas other than the area administered by the relevant base station (other cells) so that interference applied to the other cells is increased and interference from mobile stations residing within the other cells is increased.
In contrast, when the tilt angle of the antenna vertical pattern is large in the base station, the orientation direction of the main lobe becomes close to vertical so that the antenna gain toward the cell boarder is reduced and degradation due to thermal noise (coverage loss) occurs. It is noted that the above principles are disclosed in Fujii, “Optimization of Antenna Beam Tilting in Mobile Communication”, The Technical Report of the Institute of Electronics Information and Communication Engineers of Japan, RCS-292-131, January, 1993 (document 1) in relation to a system using the PDC scheme, and Hayashi et al., “Optimization of Base Station Antenna Vertical Half Angle and Tilt Angle in Cellular Mobile Communication”, The Technical Report of the Institute of Electronics Information and Communication Engineers of Japan, B-5-35, March, 2003 (document 2) in relation to a system using the W-CDMA scheme.
Also, a method of manually designing an appropriate type of base station antenna (e.g., beam tilting antenna, or beam direction antenna) by simulating and estimating the wave propagation state between base stations and locations with a cell design system employing an electronic map is disclosed in Fujii et al., “Cell Design System in Mobile Communication”, NTT DoCoMo Technical Journal, Vol. 2, No. 4, January, 1995 (document 3).
Also a specific method for manually designing appropriate beam tilting based on measurement data of locations within a service area obtained through actual measurement of the locations using a measuring device is disclosed in Asakura et al., “Mobile Communication Cell Design System using Actual Propagation Data”, The Technical Report of the Institute of Electronics Information and Communication Engineers of Japan, RCS95-130, January, 1996 (document 4). Document 4 discloses that the estimation simulation of the wave propagation state may be possible based on actual measurement values in the case of changing only the tilt angle without changing the half width. Document 4 also discloses that the reception level within the service are may be improved and resistance against interference from neighboring cells using the same frequency or adjacent frequencies may be increased through manual adjustment by the cell designer to find an appropriate tilt angle through the cut and try method.
Also, document 1 discloses a method for appropriately controlling the communication quality and system capacity by automatically selecting or manually designating the appropriate beam tilting based on at least one of the condition of the service area or a command issued from the outside, the beam tilting being manually designed beforehand with respect to various conditions.
Specifically, the amount of traffic may be monitored as the condition of the service area, for example. If the traffic amount is at full load, the number of vertically stacked radiation elements of the base station antenna to which a transmission signal is to be supplied may be reduced. In this way, the half width of the vertical pattern may be increased, and the tilt angle may be increased by setting the difference in the phase conversion amount between adjacent radiation elements to the maximum.
In another aspect, the number of vertically stacked radiation elements of the base station antenna to which a transmission signal is supplied may be gradually increased as the amount of traffic is decreased. In this way, the half width of the vertical pattern may be narrowed, and the tilt angle may be reduced by gradually decreasing the amount of phase conversion between adjacent radiation elements.
As can be appreciated from the above description, the vertical pattern may be formed to secure a larger system capacity in a cell with a small cell radius and small coverage as the traffic amount is increased. Also, the vertical pattern may be formed to secure a smaller system capacity in a cell with a large cell radius and large cell coverage as the traffic amount is decreased.
In this way, the system capacity and coverage as a whole may be improved in accordance with changes in the service area. However, it is noted that the optimum beam tilting for the each of the different conditions of the service area has to be manually designed by the cell designer beforehand so that the appropriate beam tilting may be suitably selected.
However, the techniques described above have the following drawbacks.
It is noted that street micro cells (with an area radius of no more than 1 km) and pico cells (with an area radius of no more than 0.5 km), which are further reduced in coverage area with respect to macro cells (with an area radius of no more than 10 km) and micro cells (with an area radius of no more than 3 km) corresponding to the current mainstream cells, are expected to be mainstream in future systems.
In such smaller cells, it is quite difficult to manually design appropriate beam tilting or beam directing by simulating and estimating the wave propagation between locations and base stations or actually taking measurements of locations within the service area with a measuring device according to the cell design systems disclosed in the above cited documents.
Specifically, in micro and macro cells, wide cell coverage is realized by arranging an antenna at a high position such as a steel tower, the top of a high rise building, or the peak of a mountain, for example. In this case, the propagation distance to the cell border is relatively long so that influences from individual buildings for each direction viewed from a site may be statistically averaged out. Therefore, an area with a smooth area edge forming a hexagonal shape around a site may be formed.
In this case, the propagation loss within a cell may be accurately estimated by a propagation estimation formula that is defined for each of generalized city structures. For example, with the aid of a cell design system that takes into account the general city structure based on an electronic map, appropriate beam tilting or beam directing may be designed by simulating and estimating wave propagation between locations and base stations.
Also, it is noted that in macro and micro cells, since the area radius is large, influence from individual buildings may be statistically averaged out. Therefore, representative data may be acquired by taking measurements of a portion of the corresponding area rather than extensively measuring all locations within the area (e.g., roads, squares) with a measuring device, and appropriate beam tilting or beam directing may be designed based on the acquired data.
On the other hand, in street micro cells or pico cells, small cell coverage is realized by arranging an antenna at a position lower than the buildings surrounding the base station. In this case, the propagation distance to an area edge is relatively short so that the cell edge with respect to each direction from the base station is easily influenced by individual buildings and land objects and a cell with a complex shape (amoeba shape) with a discontinuous area edge may be formed.
In this case, it is difficult to accurately estimate the wave propagation within the area based on a propagation estimation formula defined for each of generalized city structures. In other words, it is difficult to accurately design beam tilting and beam directing using a cell design system.
It is noted that a cell design system that estimates propagation loss through ray tracing allows large-scale calculation, and may thereby be capable of taking into account individual buildings and land objects upon estimation of the wave propagation. However, an electronic map cannot completely describe the actual environment such as detailed features of buildings, and information of the electronic map cannot represent the actual environment in real time since the actual environment is subject to change over time. Therefore, estimations based on the electronic map may not be adequately accurate.
Also, in a system that designs beam tilting and beam directing based on actual measurements obtained using a measuring device, accurate beam tilting and beam directing may not be designed unless a large portion of the service area (e.g., roads, squares) has to be measured in detail owing to the complexity of the area configuration.
Further, the number of base stations to be stationed may be significantly increased in accordance with the reduction in size of the cells, and in turn, design procedures to be executed by the cell designer for designing beam tilting and beam directing may be increased and more complicated.